Prima facie is a Latin expression meaning "on its first appearance", or "by first instance". It is used in modern legal English to signify that on first examination, a matter appears to be self-evident from the facts. In common law jurisdictions, prima facie denotes evidence that (unless rebutted) would be sufficient to prove a particular proposition or fact.
Most legal proceedings require a prima facie case to exist, following which proceedings may then commence to test it, and create a ruling.
For example, in a criminal prosecution, the prosecution has the burden of presenting prima facie evidence of each element of the crime charged. In a murder case, this would include evidence that the defendant's act caused the victim's death, and evidence that the defendant acted with malice aforethought. If the prosecution were to fail to introduce such evidence, then its case would fail on grounds of "failure to make out a prima facie case," even without rebuttal by the defendant. This evidence need not be conclusive or irrefutable, and evidence rebutting the case may not be considered.
The difference between the two is that prima facie is a term meaning the matter seems obvious and self-explanatory. Res ipsa loquitur is the legal argument that because it is so obvious, the plaintiff can stop their explanation there and does not have to provide any further in-depth details to prove liability, because it "speaks for itself". Example:
The phrase is very commonly used, in exactly the same sense, in academic philosophy as well. Among the most notable uses of it in that discipline is the theory of ethics first proposed by W. D. Ross, often called the Ethic of Prima Facie Duties, as well as in epistemology, as used, e.g. by Robert Audi. It is generally used in reference to an obligation. "I have a prima facie obligation to keep my promise and meet my friend" means that I am under an obligation but this may yield to a more pressing duty. A more modern usage prefers the title ‘pro tanto obligation’: an obligation inasmuch as there is this or that aspect of the situation, but again suspending the all-in verdict.